An optical fiber cannot be bent around a sharp angle because it will break and it will lose light because some of the light rays it is carrying will strike the cladding at less than the critical angle. The critical angle of an optical fiber is the angle of rays to the normal to the boundary of the fiber at which total internal reflection (TIR) begins to fail, so that the light internal to the fiber begins to exit through the cladding. When this occurs, the efficiency of transmission is significantly reduced. To turn a sharp corner and yet prevent substantial failure of TIR, the fiber can be cut and flat mirrors along with other optics can be used to redirect the light from one substantially-straight section of fiber to another substantially-straight section of fiber which is oriented in another direction. This method of redirecting light provides a break in the optical fiber and allows the light to be turned so it can again be sent down a second optical fiber or otherwise used.
A fiber optic sharp corner turner is desirable for certain applications of pulse oximetry where wires are undesirable, such as in Magnetic Resonance Interference (MRI) applications. Pulse oximetry is typically used to measure various blood flow characteristics including, but not limited to, the blood-oxygen saturation of hemoglobin in arterial blood, the volume of individual blood pulsations supplying the tissue, and the rate of blood pulsations corresponding to each heartbeat of a patient. Measurement of these characteristics has been accomplished by use of a non-invasive sensor which passes light through a portion of the patient's tissue where blood perfuses the tissue and photoelectrically senses the absorption of light in such tissue. The amount of light absorbed is then used to calculate the amount of blood constituent being measured. For measuring blood oxygen levels, sensors have been provided with light sources and photodetectors that are adapted to operate at two different wavelengths, in accordance with known techniques for measuring blood oxygen saturation using optical signals carried by optical fibers.
MRI exams are typically used to view the internal structure of the human body. Observation of these internal structures has been accomplished by use of a non-invasive magnetic resonance frequency which passes through the body to create an internal picture of the person. For measuring blood oxygen levels during these procedures, fiber-optic-coupled pulse oximeters have been provided since it is desirable in the MRI environment to avoid attaching metal wires to the patient. One reason for this is that the patient might be burned by the wires during the procedure, since the high electromagnetic fields of the MRI instrument can induce high currents in the wires. Ordinary pulse oximetry systems include semiconductor diodes closely-coupled to the patient (two or more of such diodes for light emission and at least one for light detection) and electrical wires to connect those semiconductor diodes to the instrument. A fiber optic pulse oximetry system, on the other hand, has light emission and detection means remote to the patient, and connects these means to the patient's tissue by way of optical fibers. Such a pulse oximeter enables physicians to take data without risk of harming the patient. In a fiber-coupled pulse oximeter, the “sensor” (that is attached to the patient), rather than comprising light emitting and detecting devices, may simply comprise an optical interface between a pair of optical fiber bundles and the patient's tissue, one fiber bundle for bringing light to the patient and one for carrying it away.
A pulse oximeter sensor may be attached at a number of locations, such as a finger. To avoid movement causing stress at the connection to the sensor, the connecting wires, fibers, or fiber bundles may be taped to the patient a short distance from the attachment site. This means that, if an optical fiber is used as a connector, it must run parallel to the patient, then make a sharp turn to deliver the light to the skin. A similar sharp turn would be required for a fetal application of pulse oximetry, where the connector needs to enter through the vagina, then may need to make a turn to direct light at the fetus. Other medical sensors may have similar requirements—for example, in a fiber-optic instrument designed to detect the vitality of dental pulp by sending light through a tooth, it is necessary for an optical fiber to enter the mouth, and for light leaving the end of the fiber to be redirected sharply so as to enter the tooth.
There are prior art optical devices which can achieve efficient corner turning but they use imaging optics and tend to be expensive and bulky. Imaging optics allow a one-to-one correspondence of points on the input object in the object plane to points on the output image in the image plane. Non-imaging optics provide for transmission of light from the input plane to the output plane, without the requirement for one-to-one correspondence of object and image points. Other prior art corner turning devices often require high Numerical Aperture (NA) imaging optics and tend to be expensive and lossy.
In one oximeter system described in U.S. Pat. No. 5,537,499, a probe with a flat reflector is used to redirect light at an angle lateral to the axis of the fiber bundle. This probe incorporates Fresnel light reflections from the optical fiber and air interface, directing them laterally into the fiber and capsule enclosing the end of the fiber without secondary light reflections and refractions. In U.S. Pat. No. 5,515,468, a connector system for coupling between a fiber optic transmission line and an opto-electronic device is disclosed. With this system, light is bent around a corner using flat mirrors, internally reflecting prisms, and lenses. U.S. Pat. No. 5,343,543 provides a directional indicator and methods of use which gives a surgeon visual feedback as to the direction of radiation to be emitted from a side-firing laser fiber when the distal end of the laser fiber is obscured from observation. With such a system, the laser core has an integral tilted mirror at one end to cause the emitted beam to be at an angle to the laser axis. In U.S. Pat. No. 5,152,296, a pair of finger cuffs that include an electrocardiographic electrode, a first radiation source and detector pair for blood pressure measurement, and a second radiation source and detector pair for blood oxygenation measurement are disclosed. In this setup, an optical system with various beamsplitters, lenses, optical fibers, and beam re-directors are used.
Non-imaging optics are a type of optics which have only begun to be understood within the last few decades. The state of the art of analyzing and designing such optics as of 1989 has been summarized in W. T. Welford and R. Winston, High Collection Nonimaging Optics, Academic Press, c. 1989, which is herein incorporated by reference for all purposes. At the time of publication of this text, no non-imaging corner turners were known.
Welford and Winston described (see their page 4) a distinction between two-dimensional and three-dimensional (2D and 3D) designs of non-imaging optics, a concept which will be used herein, in a generalized fashion.
2D corner turners are bounded by more-or-less complex curves in the plane of the bend. The three-dimensional shapes of 2D corner turners are generated by moving those curves perpendicular to themselves, so that every cross-section parallel to the plane of the bend is the same curve, and every cross-section perpendicular to the plane of the bend is a rectangle. This is a first type of corner turner (In spite of the name “2D,” it is a 3 dimensional object). The upper and lower surfaces of 2D corner turners are planar reflectors. A familiar example of this type of shape, which in the language of solid geometry may be called a “generalized right cylinder,” is a curved 90 degree bend in rectangular-cross-section air conditioning duct. 2D corner turners are especially well-adapted for use with input and output optical beams having rectangular cross sections, although at some cost in dilution (see definition below) they may be used with input and output beams of circular or other cross sections.
3D corner turners may in general have differently-shaped curves in each cross section in the plane of the bend, and in each cross section perpendicular to that plane. This is a second type of corner turner. Their input and output cross sections may be adapted to the shape of associated input and output optics, which will in many cases be circular. A familiar shape of the general class to which 3D corner turners belong is a right-angle bend in round copper water tubing.
An article by Collares-Pereira et al., entitled “Redirecting Concentrated Radiation,” Proc. SPIE, vol. 2538, pp. 131–135 (August 1995), describes a toroidal corner turner. This 3D device is thought by the present inventor to be the only previous published example of non-imaging optics for corner turning. The corner turner described, by its nature, is restricted to working efficiently with input and output beams, each of which has a maximum ray propagation angle of 90 degrees with respect to the beam axis (in air this corresponds to NA=1.0). If the input beam has less than a 90 degree divergence, the output beam may still have a 90 degree divergence. As such, this use of the toroid is inefficient, which is to say that it does not minimize étendue of the output beam (étendue is defined, in the 2D case, as the product of NA and the beam diameter). In an application in which a toroidal corner turner was used to direct light from one optical fiber to another, both fibers having NA=0.5, much of the light entering the second fiber would be at angles exceeding NA=0.5, and would therefore quickly be lost as it propagated down the length of the second fiber.
Consequently, a device is needed which conserves étendue while redirecting light around a corner that does not use imaging optics and can work efficiently with smaller input divergence than 90 degrees.